Spectroscopic measurements of photothermographic systems at development temperatures show how Ag particles form during the course of the development process. In standard systems, development follows autocatalytic kinetics. Systems containing infectious developers exhibit more unusual kinetics which do not generally fit simple mathematical models. In standard photothermographic systems, Ag particles have a median diameter of approximately 0.065 μm. The median diameter is reduced to 0.028 μm and the particles are generally more spherical in the presence of an infectious developer. Intermediate sizes and morphologies are possible with low levels of infectious development. Particle size and morphology correlates to light absorption and Ag covering power. Standard photothermographic development proceeds sequentially, while infectious development occurs in parallel.